In recent years, in manufacturing processes of steel products, in terms of improving yield through prevention of mass incompatibility, detection of surface defects of hot or cold steel materials has been demanded. Steel materials referred to herein mean: steel products including steel sheets and shaped steel such as seamless steel pipes, welded steel pipes, hot rolled steel sheets, cold rolled steel sheets, and thick plates; and semimanufactures such as slabs produced when these steel products are manufactured. Thus, a method has been proposed as a method of detecting a surface defect of a steel material, the method in which: a billet in a process of manufacturing a seamless steel pipe is irradiated with light; reflected light is received therefrom; and presence or absence of a surface defect is determined according to light quantity of the reflected light (see Japanese Patent Application Laid-open No. 11-037949). Further, a method has also been proposed in which: visible light beams of plural wavelength regions, which do not have mutual influence with emitted light radiated from a hot steel material and do not influence each other, are emitted from diagonal directions symmetrical to each other about a normal line of a surface of the hot steel material; an image by combined reflected light beams and images due to the individual reflected light beams are obtained in the normal direction of the surface of the hot steel material; and a surface defect of the hot steel material is detected from a combination of these images (see Japanese Patent Application Laid-open No. 59-052735).
According to the method described in JP '949, since reflectivity of a harmless pattern or scale is different from reflectivity of a base steel portion, the sound harmless pattern or scale may be erroneously detected as a surface defect. Therefore, in the method described in JP '949, by utilizing the fact that the shape of the defect is linear, the defect and the scale are distinguished from each other. However, surface defects on a steel material not only have linear shapes, but also various shapes such as circular shapes. Therefore, the method described in JP '949 is difficult to be applied to detection processing for a surface defect of a steel material. In the method described in JP '735, since there are a vast variety of defects, scale, harmless patterns and the like, scale or a harmless pattern is difficult to be distinguished from a surface defect just by a simple combination of images. Further, realistically, detection logic corresponding to the vast number of combinations of images is difficult to be constructed.
It could therefore be helpful to provide a surface defect detecting method and a surface defect detecting apparatus that enable scale or a harmless pattern to be accurately distinguished from a surface defect.